Surface Shaders and Motion Blur

I'm using a motion blurring technique I was told "the pros" use to offset mental ray's tendency to go real slow in that department. But something's not working.

The technique, as I understand it, involves rendering out the animation (in this case a camera move) without any blurring, at a resolution 10% larger than what you'll eventually need.

That done, disconnect all shaders from the scene. Give them all a single surface shader. Into its out colour, put the unblurred image sequence "as projection" from the rendering camera's perspective. Render with full motion blur. No raytrace, no shadows, no final gather.

They say it's more accurate than 2D blurring, and faster than 3D blurring with mental ray.

And it is lightning fast. But no blur appears. None. As if motion blur was set to none. I can't figure it out. It's my first time using a surface shader for anything.

There you have it. The surface shader has nothing to do with it. Motion blur simply won't work on any texture, procedural or not, with or without motion blur checked off in the render stats. Strange. I could swear mental ray USED to be able to motion blur.